This invention relates to an improved process of preparing TiO.sub.2 pigments which are stable, when dispersed in polymeric carriers, to heat, light and chemical attack.
Titanium dioxide pigments in various forms have been, for decades, the principal white pigment of world commerce.
For example, in the U.S. alone annual shipments of TiO.sub.2 pigments have exceeded 1.2 billion pounds throughout this decade, and projections of shipments in 1980 range from a low projection of 1.8 billion pounds to a high projection of 2.5 billion pounds. With this amount of product throughput it will be understood that seemingly small changes in process operating efficiency will have a favorable impact on energy and capital requirements of the TiO.sub.2 manufacturers if they are to meet the needs of the marketplace.
TiO.sub.2 pigments are used in a number of end-use applications, but the most demanding end-use application from the standpoint of product integrity are outdoor paint systems and plastics processed at high temperatures. TiO.sub.2 pigments employed in such applications must possess outstanding durability in addition to opacity, hiding power, and dispersibility. Such durable TiO.sub.2 pigments at present account for more than 50,000 tons of volume of annual usage and require far more time and investment than grades produced for less demanding applications.
It is known, although the mechanism is not fully understood, that paint films containing TiO.sub.2 are degraded via a photochemical reaction. This degradation in outdoor paints results in breakdown of the films which is commonly known as chalking.
It is also known that pigmented thermoplastic shapes when processed at elevated temperatures tend to discolor when untreated TiO.sub.2 is employed as the pigment.
One successful solution to overcome this deficiency of TiO.sub.2 as a pigment when outstanding durability is a requirement has been to coat the pigment particle with a dense amorphous shell of silica. Originally silica gel was deposited on TiO.sub.2 particles and the resultant coated pigment was calcined to convert the silica into a durable coating.
In 1959 U.S. Pat. No. 2,885,366 issued to Ralph K. Iler in which it was taught that active silica could be deposited as a skin of impenetrable amorphous silica upon a number of substrates without the need for calcination.
In 1973 U.S. Pat. No. Re. 27,818 issued to Alfred J. Werner in which it was taught that a TiO.sub.2 pigment with outstanding durability, gloss, opacity and dispersibility could be prepared when not only a dense skin of amorphous silica was used but also when alumina was added as a second treatment.
The ability to produce durable pigments in required volume to satisfy a growing market need requires substantial capital investment if currently available technology is to be used for each of the known methods for producing a pigment of high durability and requires long treatment times. The subject invention will allow a TiO.sub.2 manufacturer to greatly forshorten the treatment time and thereby increase product throughput without the need for expensive capital improvements.